Multi-stage flash distillation plant

ABSTRACT

The invention disclosed is for a multi-stage flash evaporator having a plurality of flash chambers separated by partitioning walls wherein seawater is flashed under lower pressure in each subsequent stage after the first stage. A seawater flow path connecting each adjacent pair of flash chambers is provided near its open end on the low-pressure side with means having a bellows-like or accordion-shaped passage having a cross-sectional area smaller than the flow path so as to flash seawater.

Apnl 2, 1974 KAzuo sATo ETAI- 3,301,471

MULTI-STAGE FLASH DSTILLATION PLANT Filed Feb. a, 1971 s sheets-sheet 1 INVENTORS KALULO SHTO KENJI Mamiya/1H BY KO ICHI' THF/HRH HGENT pril 2, 1974 KAzuo SATO ETAI- MULTI-STAGE FLASH DISTILLTION PLANT 3 Sheets-Sheet 2 Filed Feb.

INVENTORS snTo HGENT April 2, 1974 KAZUO SAT() ETAL 3,801,47

MULTI-STAGE FLASH DISTILLATION PLANT Filed Feb. 8, 1971 3 Sheets-Sheet 5 Non-equilibrium Loss (C) 0.1 0 I IIIIIII V0 por temperature (C) INVENTORS KHZU-O SHTO KEN-TI KIQ/"ll y19/VIH BY KCl/CHI TAI-[HRH KM XW HG ENT United States Patent() MULTI-STAGE FLASH DISTILLATION PLANT Kazuo Sato and Kenji Kamiyama, Tokyo, and Koichi f 'Tahara`, Kanagawa-ken, Japan, assignors to Agency of Industrial Science & Technology, Tokyo, Japan i 4 .i f Filed Feb. 8, 1971, Ser. No.A 113,424 :1.a Claims priority, appligz/xtioldlzapan, Feb. 12, 1970,

ABSTRACT F THE DISCLOSURE l The inventiondisclosed is for a multi-stage flash evapo` rtor vhaving a plurality of ilash chambers separated by partitioning walls wherein seawater is ashed under lower pressure in'each subsequent stage after the iirst stage. A seawater flow path connecting each adjacent pair of flash chambersis provided near its open end on the low-pres-` sure side with means having a bellows-like or accordionshapedpassage having a cross-sectional area smallerthau the llow path so as to ash seawater.

i, -This invention relates to a multi-stage liash evaporator. Thermodynamically, the seawater desalination plant using the flash evaporation method requires a larger amount of energy for the production of a unit weight of fresh water than the plant using either the refrigeration method or the reverse osmosis method. Nevertheless, this plant `is advantageous in that it is operable with 1owgrade energy such as waste heat occurring in thermal power generation and, because the liuids to be handled are in liquid and gaseous states, it permits large-scale operation and consequent reduction of operating cost. rIhese and other advantages have encouraged developmental research on such systems.-

This invention provides a multi-stage Hash evaporator capable of producing fresh water in a high yield by approximating the adiabetic evaporation process to the State of theoretical rate production.

The present invention also provides a multi-stage flash evaporator which permits the heat transfer area of the condenser to be decreased in size as compared with that required in the conventional plant and, thereby, to decrease the 'cost of construction. Generally, a multi-stage ilash evaporator has been constructed such that the seawater heated to high temperature, is'first pumped into the flash chamber of the lir'ststa'geto'be flashed therein. The ash chamber of the last-` stge is kept under the lowest pressure by a steam ejector or vthe like. By virtue lof a gradual decline of pressure, rhe seawater is caused to flow in regular succession from the first-stage to the last-stage. Any given stage is maintained ,underI a prescribed pressure by means of the interstage flash means. Hydrodynamically, ilash means are broadly clas's'ifiedrunder the following: (A) an orifice or a per-Y forated plate-for reducing the cross-sectional area of the o'w' path, is disposed in the seawater iiow path connecting adjacent stages sothat the desired pressure ditferenceis produced through loss of mechanical energy; and (B) two adjacent .stagesare so positioned relatively to each-other such that there arises a static pressure difference byvirtue of which there is produced the vdesired pressure difference.

In. tlhe c ase of a ash chamber having a given number of Astages and maintained under a specificpressureby-the aforesaid means, if the seawater which is iiowing into the chamber has a temperature higher than the equilibrium', temperature corresponding to that specific pressure, it

3,801,471 Patented Apr. 2., 1974 ICC its own sensible heat and the temperature ie reduced proportionally. Generally, however, the temperature of the seawater is not reduced to a temperature low enough for it to be in equilibrium at a given pressure. That is to say, it fails` toreach thetemperature which is the sum of the saturated steam temperature existing at the pressure in question and the boiling point elevation. This phenomenon is referred to as non-equilibrium phenomenon in the process of flash evaporation, and the non-equilibrium loss is defined as the difference between the seawater temperature less the boiling point elevation on the one hand and the saturated steam pressure on the other.

The fact that the non-equilibrium loss is large implies that the amount of product water is small. In the case of a brine-recirculation type multi-stage ash evaporator, the greater part of the seawater discharged from the liash chamber of the last-stage returns to the condenser tubes in the heat-recovery section and serves the purpose of condensing steam. Since the temperature of the seawater is increased by the non-equilibrium loss, the available temperature difference of the condensers in each of the stages becomes small, making it necessary to enlarge the heat transfer area required. It is, therefore, necessary to reduce this non-equilibrium loss to the fullest possible extent.

' A review of the aforementioned two designs of flash means reveals that the design (B') has a higher liquid level and, therefore, provides a higher non-equilibrium loss than the design (A). With this in mind, the relation between the non-equilibrium loss and the operating conditions was examined with respect to the orifice pattern in the design of (A). Consequently, it was discovered that in operating the multi-stage ash evaporator, the interstage pressure difference can be calculated in advance as an operating condition. Where the pressure loss occurring during the passage of the seawater through the orifice is equal to this pressure difference, ash evaporation begins immediately after the seawater has completed its passage through the orifice. When the cross-section at the opening end of theiorifice is enlarged without changing the ow rate, flash. evaporation begins to occur inside the flow path preceding theorice although the liquid level of the chamber is higher. At the very point of time at which the seawater is passing through the orifice, there consequently occurs atwo-phase flow of seawater and vapor.

e Experiment has shown that the non-equilibrium loss gradually decreases when the cross-sectional area at the opening end of the orifice alone is increased while the ilow rate of fluid is kept constant. At the evaporation temperature of` 60 C., for example, the non-equilibrium loss was 0.61ov C. for the standard cross-sectional area of orifice. When the cross-sectional area was doubled, however, the loss .fell to 0.43 C. This decline of nonequilibrium loss occurred because in the latter case, there was provided a longer contact time between vapor and seawater which consequently permitted both heat and mass transfers to occur at higher rates than with the former case.

` iigurationor one having an accordion-shaped flow path,

as contrasted to prior art flash means having conventional orifices. This invention will be explained in greater detail with referencefto the accompanying drawings wherein:

FIG. 1is a side sectional view of a flash chamber in the multi-stage Hash evaporator according to the present invention.

' FIG. v2A is a'plane view of the evaporator taken along line II-II of FIG. 1.

` FIG. 3 is a plan "view illustrating another embo`diment"` of the flash means according to the present invention.

FIG. 4 is a graph illustrating the results of the measure-` ment of non-equilibrium loss obtained when' the 'seawater was ashedby the conventional single-orificetype 'flash means and by the flash means of the present 'inventionl Y Referringto lFIG. 1 and FIG. 2, the seawater 1 which has been suctioned from the flash chamber of the preceding stage undergoes a pressure loss of a' prescribed magnitude determined by 3 and, thereafter Yundergoes flash evaporationV in the flash-chamber y2. The resulting two phase current of flashed" seawater and'vapor is sent up temporarily in the auxiliary flash chamber 2 separated by the baffle plate 4 from the flash chamber 2 and immediately thereafter falls to the liquid below. The sea'- water and the vapor which have fallen onto'the liquid surface flow through the opening formed at the lower end of the bale plate 4 and reach the ash chamberfZfThen', the vapor ascends and reaches the demister,5',7wherein"it is deprived of liquid droplets which are entrained thereby. Subsequently, the vapor reaches the condenser chamber y6, wherein it is cooled into a condensate by the cooling seawater which is flowing through the condenser` tubes of the chamber 6. The condensate is collected ,in the product tray 7 and then driven forth to the productray' of the subsequent stage 11 by virtue of pressure d ifferericefOn the bottom of the ash chamber 2, a weir 9 isprovided optionallyif desired. A`

The pressure ash means 3, as illustrated in FIG. 2, are formed having bellows-like passages congurated such that the cross-sectional area is varied periodically several times while having a minimum cross-section smaller than that of the ow path 8 for transferring the seawater from the flash chamber 10 to the ash chamber 2. These flash means 3 having bellows-like passages are located on the low-pressure side of flash chamber 2 and near the end of the ow path. The open end of the ow path may alternately be constructed in an accordion shape so as tovary the cross-section as illustrated in IFIG. 3. As the seawater ows out of the flash chamber 10 and reaches the lfirst constriction formed in the bellows like passage, the kinetic energy increases and the static pressure decreases as indicated by Bernoullis formula. If the static pressure is suiciently small as compared with the pressure of the liash chamber 10, the'flash evaporation begins to occur immediately at that'po'sitiom'with the consequence that vapor bubbles are'formed in the seawater to give rise to a two-phase uid of seawater and vapor. At the subsequent portion of an increased'crosssection, the two-phase ow has its static pressure `iii-` creased, though not to the extent of destroying all the vapor bubbles in existence. As the two-phasefluid iiows into the second constriction, the static pressure `is again reduced and the tiash evaporation is promotedproportionally. At this point, new vapor cores are formed and;` at the same time, vapor bubbles which are already in existence grow in size. By repeating this process by'means of a required number of constrictions incorporatedin'the bellows like passage, the flashing rate is increase'dand the contact time between vapor and seawater is lengthened,` thus permitting the flash evaporation process-to approach more closely the state of equilibrium. In other. words,'the number of such constrictions is so iixed thatwthe flash evaporation process of seawater will approximate-the state of equilibrium. v .Y l In a separate experiment, the seawater Wasilashed by-I using the ash means, at two locations ofthesingle.

This graph shows the relations between the lvapor tem-l perature and the non-equilibrium loss. It is notedthatj the temperature of the seawater owing out of the ash 4 chamber isthe sum of the saturated vapor tempeature, the boiling point elevation and the non-equilibrium loss.

In this graph, the continuous line represents the results obtained with the device of the present embodiment, wherein the ratio of the largest c'ross section to the smallest cross-,section is 5:1l arid the number of layersis 4. The dotted., line vrepresents thel results of the l.m'easurerr'1-lent made .actually by the inventors on the conventionalsingleorice type ash means, while the Valternate longY and short` dash line represents the results-of measurement which W.R. Williamson et al.of Cuno Division, American Machine & Foundry Company obtained ofthe single: orifice type device and published in the' Ofce'iof Saline Water Report N0. 525.

Comparisony of the data reveals that, at'allvapor temperatures, the values of non-equilibrium loss` obtained for the present embodiment are 1/2 yto 1/3 of those obtained for the single-orifice typeV devices. This indicates that. the flash means -of the present embodiment is highly effective inlowering the non-equilibrium loss. f y' The pressure loss which occurs at the time that the seawater passes4 this device cannot Abe calculated vunless a solution is given to :a more complicated ow equation than that available for the conventional orice type de:-l vice. However, the pressure difference required'lbet'weeii interstages and the pressure loss afforded by the present device are, in principle, expressed as follows onthe basis of Bernoullis formula:

wherein, F denotes the pressure loss expressed in terms of water head, U1 and U2 denote the flow velocity of sea: water respectively at the ash chamber on the higher pressure side and at the iiash chamber on the lower pressure side, P1 and P2 denote the pressures of the liash chamber onthe higher pressure side and that on the lower pressure sidevv respectively, L1 and L2 denote the liquid levels in the liash chamber on the higher pressure side and that on the lower pressure side respectively, p` denotes the density of seawater, g denotes the acceleration dueto gravity and gc denotes the conversion factonrBy disregardf ing the changes in U and L, the preceding expression can be simplified as follows:

' Let Uo stand for the liow velocity at the constriction of the present device, Up for the iiow velocity in the inter-V stage iiow path, A0 for the constricted cross-sectional area andl Ap-for the cross-sectional area of the interstage iiow y path, and the pressure loss F Which is directly proportional to the kinetic energy will be expressed kby the following was 1.68. In the present pressure reducing device the non-equilibrium loss can be effectively reduced'by fixing the value of A/Ap at 0.5 or less.

'l AIf the relation between the flow coefficient Ko andthe pressure loss is experimentally determined in advance with respect to the typical shape and dimen'sions'of the present device, it will permit easy drawing ofwaldetaiecf The preceding example represents a'case wherein the cross-sectional shape of the internal wall of the flow path is'varied circularly. Otherwise, the liow path maybe formed in the shape of a parabola, a sine wave, atriangular ditch, a rectangular ditch, or. the like.

`The investigation conducted by the inventors has shown that, at vthe last stage of the multi-stage ash evaporator, the non-equilibrium less obtainable for the evaporation temperature of 35 C. is 0.5 C. in the case of the device of the present invention compared with 1.2 C. by the conventional orice type device.

When the total heat transfer area of the condenser in the multi-stage llash evaporator of the `present invention is compared, on the basis of the preceding results, with that in the conventional single-orifice type multistage flash distillation plant, there are derived the following data.

It is assumed that in the plant using the device of the present invention, the inlet and outlet temperatures of the cooling seawater which is passing through the condenser tubes of each stage differ by 7 C. and 5 C. respectively from the vapor temperatures at the corresponding points. Then, in the case of the plant using the single- Oriiice type device, the differences will be lowered by the non-equilibrium loss respectively to 6.3 C. and 4.3 C. Assume the llash down per stage to be 2 C., and the elfective temperature differences at the outer surface of the condenser tubes will be 5.944 C. and 5 .237 C. respectively.

If the total amount of heat transfer and the overall heat transfer coelllcient for the two types are equal, then the ratio between the heat transfer areas is the reciprocal of the ratio between the effective temperature dillerences. Therefore, the heat transfer area of the present invention is equal to 88.1% `of heat transfer area of the single-orifice type.

In the evaporator of this invention, the cost of the condenser tubes accounts for 30.8% of the cost of fresh water production as compared with about 35% for the conventional plant.

The total llash down between the rst stage and the last stage is 0.7 C. which is higher for the present invention. From this, it follows that the total llash down in the plant of this invention is 85.7 C. where that in the singleorifice type plant is 85 C. This means that the cost of fresh water production is 0.8% greater for the plant of this invention.

The reduction in the cost of the condenser tubes and the increase in the amount of fresh water production are such that the multi-stage flash evaporator of the present invention is found to provide a 5% saving in the overall cost of the production of a unit weight of fresh water.

What is claimed is:

1. A multi-stage flash evaporator which comprises in combination, a plurality of flash evaporator stages connected in horizontal series and each partitioned by a plate, a baille plate disposed from an upper surface of each stage and dening an auxiliary llash chamber with a plate along one side of the stage and defining a llash chamber with a plate along the opposite side of the stage, said llash chamber having a demister, a condenser and a product tray all disposed at the top thereof, said demister disposed between the baffle plate and the plate along said opposite side of the stage and below the condenser and product tray, and a Weir disposed at the bottom thereof to prevent short passage of vapor; a seawater llow path means leading from the bottom of a llash chamber of one evaporator stage and to the auxiliary llash chamber of an adjacent llash evaporator stage at the bottom thereof with llash means disposed in the Water llow path along the bottom of the stage, said llash means having a bellows conliguration.

2. The multi-stage flash evaporator according to claim 1, wherein the cross-sectional shape of the llow path for transferring seawater from one llash stage to said llash means is rectangular.

References Cited UNITED STATES PATENTS 3,180,805 4/1965 Chirco 159-2 MS 3,619,378 11/1971 Ricard 202-236 3,203,464 8/1965 Kingma 202-173 3,461,038 8/1969 Lind 202--173 3,427,227 2/ 1969 Chamberlin 202-173 3,488,260 1/1970 Gilbert 2oz-173 2,908,618 10/1959 Bethon 203-11 2,759,882 8/ 1956 Worthen et al. 202-174 3,630,851 12/ 1971 Kawaguchi et a1 202-173 WllBUR L. BASCOMB, JR., Primary Examiner U.S. Cl. X.R. 

